Lubricated oxymethylene composition

ABSTRACT

AN OXYMETHYLENE COMPOSITION LUBRICATED WITH A FATTY ESTER DERIVED FROM A FATTY ACUD AND A POLYHYDRIC ALCOHOL.

United States Patent O 3,808,133 LUBRICATED OXYMETHYLENE COMPOSITION NorthropBrown, Wilmington, Del., assignor to E. I. du Pont de Nemours and Company, Wilmington, Del. No Drawing. Continuation-impart of abandoned application Ser. No. 201,133, Nov. 22, 1971. This application Feb. 28, 1972, Ser. No. 230,047

Int. Cl. C10m /12 US. Cl. 252--56 R 13 Claims ABSTRACT OF THE DISCLOSURE An oxymethylene composition lubricated with a fatty ester derived from a fatty acid and a polyhydric alcohol.

PRIOR APPLICATION This application is a continuation-in-part of US. application Ser. No. 201,133, filed Nov. 22, 1971, now abandoned.

SUMMARY OF THE INVENTION This invention relates to lubricated oxymethylene compositions. More specifically, this invention refers to a lubricated oxymethylene composition wherein the lubricant is a fatty ester derived from a fatty acid and a polyhydric alcohol.

BACKGROUND OF THE INVENTION Suitable additives have been sought for a number of years for blending with oxymethylene polymers so that an article fabricated therefrom would, in a bearing ap plication, exhibit a low coefiicient of friction and suffer a minimum loss of material through wear. Unfortunately, many additives are deleterious to physical properties. For example, compositions containing 5% or more of dispersed particles of polytetrafluoroethylene which have been used by the trade for many years show a reduced tensile strength and elongation in a molded article.

Interference with processibility is another problem that can result from the presence of additives having lubricating properties. For example, oxymethylene polymers containing common petroleum lubricants or mineral oils often feed poorly through an extruder or molding machine and will not blend properly.

Other additives, such as stearyl alcohol, may impart initial lubricity. However, such lubricity disappears in time or after heating because of volatilization or exudation of the additive.

DETAILED DESCRIPTION OF THE INVENTION This invention is the discovery of a lubricated oxymethylene composition containing an oxymethylene polymer having at least 80 mol percent oxymethylene units and 0.3 to 5 weight percent based on the weight of the polymer of a fatty ester derived from at least one saturated or unsaturated fatty acid of 12 through 20 carbon atoms and at least one polyhydric alcohol. These readily processible compositions exhibit a long lasting low coefficient of friction and low wear without significant deleterious effect on their physical properties as compared to corresponding unlubricated compositions.

The oxymethylene polymer can be a copolymer of 80 mol percent or more oxymethylene units and up to 20 mol percent of a second comonomer unit or it can be a homopolymer having substantially all CH O units. Copolymers are described in US. Pat. 3,027,352 and typical homopolymers are described in U.S. Pats. 2,998,409 and 3,161,616. The number average molecular weight of the oxymethylene polymer used in this invention should be higher than 15,000.

3,808,133 Patented Apr. 30, 1974 Typical of the fatty acids used to make the fatty ester are lauric acid, tridecylic acid, myristic acid, pentadecylic acid, palmitic acid, margaric acid, stearic acid, nonadecylic acid, arachidic acid, oleic acid, elaidic acid and closely related naturally occurring fatty acids or mixtures thereof.

The polyhydric alcohol used to make the fatty ester can be any one or more of the following: ethylene glycol, propylene glycol, homopolymers of ethylene glycol and propylene glycol or copolymers thereof with an average degree of polymerization through 15, as well as glycerol, diglycerol, pentaerythritol, trimethylolmethane, trimethylolethane and trimethylolpropane.

The lubricant is added to the copolymer or homopolymer before, during or after the addition of conventional additives such as antioxidants, stabilizers, pigments, mold release compounds, plasticizers and fillers. Mixing is carried out in a high shear device such as an extruder. The lubricant also can be dry blended with a previously extruded molding powder.

Before addition of any additives, the oxymethylene homopolymer should be stabilized by being either acetate capped or alkyl capped in accordance with customary procedures as shown in US. Pat. 2,998,409 and US. Pat. 3,161,616.

The amount of fatty ester lubricant added to the oxymethylene polymer, based on the weight of the polymer, is 0.3 to 5 weight percent. A preferred amount of fatty ester is 0.5 to 1.5 weight percent because of the resulting favorable balance of mechanical and lubricating properties in the polymer composition. Amounts greater than 1.5 weight percent tend to adversely affect their mechanical properties. About 1% by weight fatty ester lubricant based on the weight of the polymer is most preferred.

At the higher concentrations of lubricant a penalty in processibility may be encountered; i.e., a decrease in output from the extruder used for mixing the lubricant and the polymer. In such a case the addition of a quantity in the range of 0.2 to 1.0 weight percent of finely divided particles of a copolymer of tetrafiuoroethylene (TFE) and hexafluoropropylene (HFP), made in the manner de scribed in the claims of US. Pat. 2,946,763, can give improvement in the rate of extrusion or molding.

In order to test a molded article for a satisfactory lubricating property, a friction test is used to determine the articles coefficient of friction. The procedure for determining the coefficient of friction of the molded articles made from the compositions of this invention is a modification of the test set forth in the article entitled, Wear of Plastics-Evaluation for Engineering Application by Robert Bee Lewis, appearing as paper No. 63- WA-325, of the Journal of the American Society of Mechanical Engineers (ASME).

FIG. 5 of this ASME article shows a diagram ,of a thrust washer tester. For the friction test used in the examples below, sample holders are modified so that the upper sample holder contains the molded film to be tested. The lower sample holder is a circular block of Vespel polyimide resin which is 5.239 cm. in diameter with a center hole 1.905 cm. in diameter and intended to hold a carbon steel wear ring which has an outer diameter of 3.175 cm., an inner diameter of 2.381 cm. and an average radius of 1,389 cm. The wear ring is held in place with set screws.

A thermocouple measures the sample temperature in the wear ring. The friction torque arm has a strain gauge positioned 12.70 cm. from the axis supporting the wear ring to measure torque. The load exerted on the molded film is 6.35 kg. and the film rotates at a speed of 240 rpm.

Each molded film is made by compression molding the polymer composition at C. at 18,100 kg. ram pressure in a laboratory press (Model P-215C, Pasadena Hydraulics Inc.) using an aluminum chase 0.127 mm. thick. The platens are cooled by circulating water while maintaining pressure. Each molded film is measured with a micrometer to allow a maximum variation of 0.025 mm. Each molded film sample is allowed to stand overnight;

5 scribed in Example 1. One film is tested as molded and it is rinsed with acetone aid air dried before testing. the other is aged for forty hours at 120 C. in air. The co- .Screening tests indicate that the permanence of an addiefficient of friction results for the two films are as f0ltive (resistance to aging) can be conveniently evaluated lows. gz g g g g 2 film P f to a an. oven i Coefficient of friction of non-aged film (as molded) 0.20 OmParaPve ests on ms Suggest or Coefficient of friction of aged film 0.24 satisfatcory properties in end-use products made from compositions of this invention, a film after aging 40 EXAMPLE 11 houfs at should 00min exhibit a low Wear test data is also obtained, for two injected molded effiment of masured by thls test discs prepared from a like composition as in Example 1,

The wear 15 Prepared by manually Pollshmg m 15 according to the ASME article. The wear test is perrandom directions with Norton-600A Carborundum paper formed in the manner Set forth in the ASME article or the equivalent. The wear ring is then rinsed with acedescrihgd above except the wear ring is polished with tone and dried thoroughly to remove all abrasive particles. NOrtoh 6QOA Carbomndum Paper to a polish Smoother Each molded film is rotated for ninety minutes against than 16 microinches specified in the ASME article. The the wear ring and the coefiicient of friction is determined 20 wear data is as follows. from data taken at the end of the time interval in accordahce with the f ll i equation. Wear factor 10 (cu. m. min/ft. lb. hr.)

not a ed 7 Coefiicient of friction (i 2225}: aged 5 c 29 m kilograms from stmm gaugex 1270 A comparison is made with two molded discs made in the 1389 same manner but using 1% by weight stearyl alcohol instead of of ethylene glycol distearate and as a control a a s ff gg examp 165 Set forth the mventlon m sun third and fourth disc made from a like composition as in EXAMPLE 1 30 Example 1 but not containing any lubricating additive.

The wear test data for these samples is as follows. A composition containing acetate capped oxymethylene homopolymer having a number average molecular weight of about 38,000, 0.2% by weight 2.2'-methylene-bis(4- S 1e ed methyl-6-t-butylphenol) and 0.75% by weight nylon ter Sample at? i 58 C for P ymer 6/66/610 (38%/35%/27%) stabilizer is ad- Additive imaged 401mm mixed in a high shear mixer (2 inch Sterling extruder) 1% stemmwhol 2 4 with 1% by weight ethylene glycol distearate. The com- Con 01 g Position is then eXtruded and pressed into a 4,445 diameter by 0.127 mm. thick film. This film is tested for 40 It is self-evident from this data that the composition confgi ggi g of fnctlon' The coefliclent of mctlon found taining the stearyl alcohol loses its lubricity upon aging i film prepared from the same composition in unlike the composition containing ethylene glycol distealike manner is then aged for forty hours at 120 C. in air rate EXAMPLE 12 and then allowed to stand overnight at room temperature. The coefiicient of friction test is repeated. This film Molded articles made from the composition of Example sample has the same coefficient of friction, 0.18. 1 are tested for physical properties as compared :oba like ox meth lene homopolymer not containing any u ricat- EXAMPLES 2 9 ing addil ive. The results indicate no significant loss in Example 1 is repeated in all the following Examples physical properties caused by the addition of 1% ethylene 2-8 except that a like amount by weight of the indicated l l di fatty acid ester is substituted for the ethylene glycol distearate of Example 1. For Example 9, six control samples oxymethylene homopolymer,- are made in the same manner as the film of Example 1 f$fi but without addition of any fatty acid ester. The coeffi- (4-metliyl-6-t-butylphenol) cient of friction of each molded film is measured and the and g? results indicated a +1% ethylene glycol Coefficient of- Control distearate Frietl otn gg F3333 Etigilifiiii iiti i2355:3331::: ig; Ex. Fatty acid ester molded) film Z0 mpac s'ln a g mygfylmstemm 3-33 It is expected that the lubricants of this invention will 4: do: "(iii likewise provide satisfactory lubricating properties to co- 2: (31:72:51 monosmmte 8 polymers containing at least 80 mol percent (-CH O-) 7 Polyiethylene glycol 400monostearata- 0.27 "6.55 Units. 8: No fa lzty acid ester (range of six con- 0. 45-0. 68 EXAMPLE 13 trol films).

A composition containing an oxymethylene copolymer 70 containing about 2 mole percent of --0CH CH groups EXAMPLE 10 in the chain and 0.41% 2,2'methylene-bis(4-methyl-6-t- A composition is prepared as in Example 1 except that butylphenol) and 0.06% dicyandiamide is admixed in a a like amount by weight of diethylene glycol distearate is high shear mixer with 1% by weight of ethylene glycol substituted for the ethylene glycol distearate and there is distearate. The composition is pressed into a film 4.445 added 0.5% by weight, based on the weight of oxymethylcm. diameter by 0.127 mm. thick, and the film is aged for forty hours at 120 C. in air as in Example 1. The coefficient of friction in duplicate samples is determined to be 0.25 and 0.27.

From a copolymer composition the same as the above, except containing no ethylene glycol distearate, is pressed a film which is aged as above. The coefficient of friction of the aged film is 0.41.

What is claimed is:

1. A lubricated oxymethylene composition comprising a major amount of an oxymethylene polymer having a molecular weight above 15,000, said polymer having at least 80 mol percent oxymethylene (-CH units and 0.3 to weight percent, based on the weight of polymer, of a fatty ester derived from at least one saturated or unsaturated fatty acid of 12 through 20 carbon atoms and at least one polyhydric alcohol selected from the group consisting of glycerol,

diglycerol,

pentaerythritol,

tn'methylolmethane,

trimethylolethane,

trimethylolpropane,

ethylene glycol,

propylene glycol,

homopolymers of ethylene glycol having an average degree of polymerization through 15, homopolymers of propylene glycol having an average degree of polymerization through 15, and copolymers of ethylene glycol with propylene glycol llngving an average degree of polymerization through 2. A composition according to claim 1 wherein the fatty ester is present in the range of 0.5 to 1.5 weight percent.

3. A composition according to claim 2 wherein the oxymethylene polymer is a homopolymer.

4. A composition according to claim 3 wherein the fatty ester is present at about one weight percent based on the weight of polymer present.

5. A composition according to claim 3 wherein the fatty ester is ethylene glycol distearate.

6. A composition according to claim 3 wherein the fatty ester is glyceryl tristearate.

7. A composition according to claim 3 wherein the fatty ester is glyceryl monostearate.

8. A composition according to claim 3 wherein the fatty ester is diethylene glycol distearate.

9. A composition according to claim 3 wherein the polyhydric alcohol component of the ester is ethylene ycol. g1 10. A composition according to claim 3 wherein the polyhydric alcohol component of the ester is polyethylene glycol with an average degree of polymerization from 3 to 15.

11. A composition according to claim 3 wherein the polyhydric alcohol component of the ester is propylene glycol.

12. A composition according to claim 3 wherein the fatty acid component of the ester is stearic acid.

13. A composition according to claim 3 wherein the fatty acid component of the ester is palmitic acid.

References Cited UNITED STATES PATENTS 3,164,563 1/ 1965 Maxwell et al 260-67 FP 3,516,933 6/1970 Andrews et al. 25212 2,210,140 8/1940 Colbeth 252-56 R 3,114,708 12/ 1963 Morway et al. 25212 3,287,288 11/1966 Reiling 252-12 X 3,236,929 2/ 1966 Jupa et a1 26067 3,340,219 9/ 1967 Stemmler 26018 3,578,621 5/ 1971 Stapfer 260-3 1.6

FOREIGN PATENTS 943,280 12/ 1963 Great Britain.

1,128,988 10/ 1968 Great Britain.

WARREN H. CANNON, Primary Examiner US. Cl. X.R. 

